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Carburetor Downdraft


Can anyone please tell me the permitted angle of downdraft allowed for a Mk.2 Amal concentric carburetor. I have heard that it is 14 degrees, but have seen Vincents with at least 20 degrees on the front carb. How does this affect the float height, and are the stories of them filling cylinders up with fuel, true.
Many Thanks to those of you who can provide an answer.
Phil ET43 Primmer:confused: very confused!!


Well Known and Active Forum Website User
VOC Member
Hi Phil - the answers to your questions are yes, yes and yes/maybe. According to some Amal literature I've got the maximum allowable inclination for mk2's is 20 degrees. I run my front carb at this angle and have had problems (some related to the use of alcohol). You'll need to move the pilot jet from the front of the carb float bowl to underneath the main body near the air intake (on the float/body gasket face). There is a pilot circuit blanking screw which you'll need to swap with the jet. This prevents fuel from continuously bleeding into the pilot system - much like GP2 types do.
I've been fiddling around to also see if the float height changes relative to main jet centre line but cannot draw any real conclusions here as I've still got a flooding problem as I deliver fuel at 1 psi pressure which is near 1.5 metres head. No it won't fill the engine with fuel and it will function in standard format but do move the pilot jet. I suggest that you also use the new type of floats from Amal. Hope this is helpful. Cheers David


Hi David,
What a star you are, just what I needed and at least I can now use the 15degree stub I have made to suit the 36mm carb.
Good luck with your fuel problems, and keep on thrashing that delicious outfit. I wish that I could still ride mine.
Many Thanks again, ET43
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